This invention relates to a drive control system of a servomotor for driving a load such as a machine tool.
In order to control a multiple-axis machine tool or the like with a high degree of accuracy, use is made of a servomotor driven by a command signal from a numerical control unit (hereafter referred to as an NC unit) or the like. Conventionally, servomotors of this type are driven by analog servo systems. However, with the progress that has recently been made in the technical development of computer control, various expedients dealing with the question of how to control a plurality of servomotors efficiently by a single control unit have been devised.
FIG. 2 is a block diagram illustrating the general features of such a servomotor control circuit. As shown in the Figure, a main CPU 1 of microprocessor executing predetermined processing, and a plurality of executing predetermined processing, and a plurality of shared RAMs 2, are connected to an NC unit side by a data bus B1. A servo-control CPU 3 of microprocessor construction is connected to each of the shared RAMs 2 via a bus B2 and outputs an analog signal indicative of a velocity command to each servoamplifier 4 associated with a servomotor. The servoamplifier 4 comprises a velocity control circuit 5, a current control circuit 6 and an inverter 7 and subjects a servomotor 8 to prescribed feedback control on the basis of current, position and velocity.
Each shared RAM 2 is for performing an exchange of data between the main CPU 1 and the servo-control CPU 3. For example, a signal indicative of the position of the servomotor 8 is temporarily stored in the shared RAM 2 through the servo-control CPU 3, and the main CPU 1 reads out this stored value to perform display processing and the like. Further, a position command signal from the main CPU 1 is temporarily stored in the shared RAM 2 and the servo-control CPU 3 reads out this stored value and outputs a velocity command to the servoamplifier 4.
In such digital servomotor control, it is considered to provide the servo-control CPU and the servoamplifier on the servo side, namely close to the load, such as a machine tool, driven by the servomotor. With the above control system, however, the environment in which the CPU is installed is undesirable for the electronic circuitry in view of vibration and temperature. Such an environment is a cause of failure and malfunction, especially for a CPU using an IC having a high degree of integration. Since complex circuitry for position, current and other control functions is provided on the load side, adjustments and maintenance performed by a serviceman or the like are accompanied by complications and hazards. Problems are also encountered in terms of operating efficiency.
Furthermore, since the main CPU and the CPU on the servo side are remote from each other, a high-speed interface device is necessary to transfer data. A problem that results is higher cost.